1. Field of the Invention
The invention relates to emission control valves for gas-fueled engines, both carbureted and fuel injected. More particularly it relates to an emission control valve with direct emissions sensor input having internal control and full fuel authority without supplemental fuel metering or biasing of a pneumatic pressure regulator.
2. Background Information
Emission control devices and methods for use with stationary or mobile engines are numerous and extensively described in prior art patents and literature. Such devices can be separated into two major groups based on their respective fuels. The two groups represent significantly different areas of technology, notwithstanding their common goal, since the structure and operational characteristics of the two types of applications are quite different. One group is those that are fueled with a liquid fuel, such as gasoline or diesel fuel; that group is not involved in the present invention. The other group, to which this invention is directed, is those engines and engine systems, particularly reciprocating engines, that are fueled with gaseous fuels, such as natural gas, butane and propane. (Therefore the use of the word “gas” in the specification and claims herein shall mean gaseous fuel and does not refer to gasoline.)
There have been numerous prior art devices for regulation of gas-fueled engine operation which seek to control emissions at low levels. While some have had varying degrees of success, they have primarily relied on various types of supplemental fuel metering, biasing of a pneumatic pressure regulator, or limited throttling of the main fuel supply and have required substantial amounts of external support equipment and electrical interconnections among such equipment, have suffered from slow response and generally have not been particularly easy, convenient or economical to install, operate or use. Few have had any significant degree of self-containment or full fuel authority.
One system which is currently used in basic or modified form for several commercial products is disclosed in U.S. Pat. No. 5,105,790. In this system for use with turbocharged engines a small fluid bleed unit is used in which there are a pair of restrictive orifices in series with a nozzle. Regulated fluid output pressure measured between the orifices is used to control a pneumatic pressure regulator that in turn operates on an engine fuel line to regulate fuel pressure and enhance engine efficiency. This system and others like it are susceptible to the performance deficiencies of a pneumatic pressure regulator such as droop in the set point due to spring rate and/or hysterisis. This type of system must wait for a subsequent change in the oxygen sensor reading before correcting for such errors, which results in a substantial time lag in engine response to load changes. A second system which is of greater relevance to the present invention is that disclosed in U.S. Pat. No. 6,003,543, which uses a closed loop control on a pressure transducer to maintain the pressure downstream of an electromagnetically actuated poppet valve. In this system, however, finite incremental variable control of flow is not possible; the system can operate only to fully open or fully close the poppet valve. Such operation has limited resolution and turndown ratio and often results in instability in the regulated pressure, as exemplified in the patent in a test of a proportional-integral controller. Such a system is practical only for very low flow regimes where significant pressure fluctuations are acceptable and precise pressure regulation is not necessary.
Earlier engines were designed to run with about 10% excess air. This enabled the engines to accommodate varying loads which caused a variation in fuel mixture without complex fuel controls since there was always sufficient air to burn the amount of fuel reaching the engine. However, such non-stoichiometric engines emitted substantial exhaust pollutants. As catalytic pollution emission systems became required on engines, the engines had to be operated in substantially stoichiometric air/fuel ranges, since the catalysts could not tolerate oxygen contents in the exhaust of more than 2–3%. In practice the stoichiometric engines operating with catalytic converters require a precise fuel mixture that can not be achieved over the power range of no load to full load with a pneumatic pressure regulator and a carburetor. The industry has attempted to compensate by creating fuel control systems such as those mentioned above in an effort to maintain a precise fuel mixture and the resulting low emissions. To date those fuel control systems have been, as noted above, neither simple in structure nor reliable to use, nor effective during transient speed and load changes.
It would therefore be of great interest to have an emission control device for a gas-fueled engine which would be substantially self-contained, would operate with full fuel authority without need for any supplemental fuel metering or additional pressure regulators, would be rapidly responsive, would automatically correct for pressure errors independent of oxygen sensor input, could provide stable operation over a wide range of load fluctuations, and which would be capable of maintaining precise emission control during speed and load transients.